1. Field of the Invention
The present invention relates to a dielectric waveguide radar module for use as a component in a millimeter wave radar device installed on a motor vehicle, and more particularly to a dielectric waveguide mixer for use in such a radar module.
2. Description of the Prior Art
Radar devices for use on motor vehicles such as automobiles in combination with warning units for preventing collisions are required to have a high degree of resolution for detecting objects at close distances, for example, at distances of up to about several decimeters. In view of this high-resolution requirement, FM radar systems are preferred over pulsed radar systems for use in vehicle-mounted application. Since the maximum range to a target, such as a preceding motor vehicle or an upcoming motor vehicle, that may be detected is a relatively short distance, roughly several hundred meters, it is suitable for vehicle-mounted radar systems to use radiowaves in the millimeter range. These waves have a frequency of about 60 GHz and can be attenuated greatly upon propagation in order to prevent propagation beyond a necessary range and also to minimize interference with existing microwave communications equipment. Use of millimeter waves is also preferable from the viewpoint of reducing the size of a radar module which may include an antenna, FM signal generators in front and rear stages, a mixer, and other components.
Heretofore, FM radar modules in the millimeter range have been constructed in the form of a microstrip waveguide or a waveguide. Because microstrip waveguides radiate a large amount of power, they suffer a large loss, and interference tends to result between multiple of modules. This results in a reduction in measuring accuracy. A conventional waveguide is disadvantageous in that its circuit is large in size and expensive.
One attempt to solve the above problems is illustrated by a non-radiative dielectric waveguide such as that disclosed as in the article "Millimeter wave integrated circuit using a non-radiative dielectric waveguide" written by Yoneyama et al. and published in the Journal of Electronic Information Communications Society, Vol. J 73 C-1, No. 3, pp. 87 94, March 1990. The disclosed non-radiative dielectric waveguide comprises two confronting conductive plates spaced from each other by a distance slightly smaller than a half wavelength and a rod-shaped, dielectric member inserted between the conductive plates for allowing propagations only along the rod-shaped dielectric member. The upper and lower surfaces of the non-radiative dielectric waveguide are completely shielded by the conductive plates. Since the distance between the conductive plates is shorter than a half wavelength, radiowaves are prevented from leaking laterally out of the non-radiative dielectric waveguide. Therefore, any power radiation from the non-radiative dielectric waveguide is very small, effectively avoiding interference between multiple modules and also radiation loss in each module. Various components including a directional coupler and an isolator can easily be fabricated by positioning non-radiative dielectric waveguides closely to each other or adding ferrite. Therefore, a plurality of high-frequency functional components can be fabricated between two flat metal plates, making it possible to reduce the overall size of a module to such an extent which is comparable with conventional microwave IC (MIC).
The above article also discloses transmitter and receiver structures for use in the millimeter wave band which employ non-radiative dielectric waveguides. The radar module disclosed in the above article has a problem in that its overall size is large because its transmitter and receiver are of separate structures. Further, it is difficult to reduce the-size of the disclosed radar module. This makes it difficult to dispose such a device in a door or a bumper of an automobile.
Since the transmitter and receiver of the disclosed radar module are separate structures, each of the transmitter and receiver must have a high-frequency generator of its own, making the radar module expensive. The radar module is designed as a Doppler radar using a constant wave (CW) of a constant frequency. However, such a radar arrangement cannot be applied to an FM radar system for detecting the positions (bearing and relative distance) of obstacles which may exist around the automobile.
The above article further shows a single balanced mixer. While the single balanced mixer is advantageous in that it suffers low noise, the cost of its parts is high and it is difficult to reduce the size of the module employing the same because the mixer employs two diodes of the same characteristics and requires impedance matching. In order to achieve impedance matching between the waveguide and two diodes having the same characteristics, it is necessary to carry out a complex adjusting process of varying the air gap and the thickness of a thin film having a high dielectric constant. Since it is very difficult to effect such an adjusting process, the adjusting process may produce nonuniform characteristics in radar modules and reduce the rate of mass production of radar modules. The balanced mixer requires, in principle, that a local signal be divided and supplied to the mixer diodes. Therefore, the balanced mixer should be supplied with a local signal of considerably high power in order to produce an output signal of a desired level. In the case where a homodyne radar system with a single high-frequency generator is designed, the output signal transmitted from the transmission antenna is decreased by an amount commensurate with the assigned energy, since a considerable proportion of the output signal from the generator has to be assigned to the local signal. As a consequence, the maximum distance that can be covered by the radar system is reduced.